For the untrained eye, this seems to be a fine power supply, especially when we compare its transient filtering stage with the ones used by other low-end power supplies. But when we took a closer look at it, we got scared: the design on the primary is ridiculous compared to other products available today. Instead of using a modern design using MOSFET transistors, its primary uses the same old design used by AT power supplies, using regular transistors (i.e. BJT transistors). Yes, this wasn’t a typo: we are talking about old AT power supplies. We didn’t even cover this design on our Anatomy of Switching Power Supplies tutorial, as we though nobody was still using it!

The main problem with this design is efficiency. FET transistors have high impedance, and the higher the impedance, the less power the component will draw from the circuit for its own operation – meaning less consumption and energy waste. Since this power supply uses regular transistors on its switching section, it cannot have a high efficiency – power supplies using regular transistors have a typical efficiency between 50% and 60% (Seventeam states 65% as the typical efficiency for this unit). Of course we will check the power supply efficiency during our tests.

A power supply with 60% efficiency means that 40% from what it pulls from the power grid are wasted inside the power supply. For example, if your computer is pulling 300 W from the power supply, the power supply is pulling 500 W from the AC outlet – the rest is consumed by the power supply and wasted as heat. Yes, this is very bad, meaning a higher electricity bill.

As we have already mentioned, this unit doesn't have a PFC circuit. This has no practical implications besides the fact of Seventeam not being able to sell this product in Europe. Power supplies without PFC from competing companies do not use this design anymore; all of them use MOSFET transistors using one of the configurations describe on our Anatomy of Switching Power Supplies tutorial. It is simply unthinkable of using this obsolete approach in 2008.

Well, let’s take a better look on the primary. It uses one PBU1005 rectifying bridge, which can deliver up to 10 A (rated at 100º C). No heatsink was used to cool down this component.

On the switching section two 2SC3320 NPN power transistors are used using the very same configuration used by very old AT power supplies, as we mentioned before. Each transistor has a maximum rated current of 15A @ 25º C.

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Figure 9: Two power NPN transistors are used on the switching section.

On Figure 10 you can find the schematics of a very old AT power supply. Seventeam ST-420BKV primary stage uses exactly the same schematics. The secondary is different, though and we will talk about it next.

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Figure 10: Schematics of a very old AT power supply. This power supply uses the same design on its primary.